DC Police Chief Cathy Lanier gives an update on the investigation into the Navy Yard massacre. Photograph by Benjamin Freed.

Aaron Alexis, the Navy veteran suspected of carrying yesterday’s shooting spree at the Washington Navy Yard, entered Building 197 with a valid identification pass while carrying a shotgun he purchased legally at a gun shop in Virginia, the FBI said in the latest update from law enforcement on the massacre that left 13 people dead, including the assailant.

Valerie Parlave, the FBI’s assistant director who runs the Washington field office, also said that Alexis, who was employed by an information technology firm working on a Navy intranet, arrived in the DC area around aug. 25, and stayed in a series of hotels. Most recently, he lived out of a Residence Inn about a mile and a half from Navy Yard. Parlave said Alexis, 34, checked into the hotel on Sept. 7.

Alexis, who was an engineer’s mate in the Navy between 2007 and 2011, had a security clearance through his employer. He passed a background check to work on military projects, although his personal history shows multiple run-ins with police over the years and that he sought treatment from the Veterans’ Administration for mental illness.

“Mr. Alexis had legitimate access to the Navy Yard as a result of his work as a contractor, and he utilized a valid pass to gain entry into the building,” Parlave said.

Parlave and other law enforcement officials also put to rest again reports that Alexis wielded an AR-15 semiautomatic rifle, similar to weapons used to carry out other mass shootings around the United States.

But what exactly set off Monday’s bloody scene—the deadliest single day in DC’s history since a plane crashed into the 14th Street Bridge in 1982—is still very much unknown. Parlave said the FBI is investigating leads with Alexis’s friends and family in every location he is known to have lived in, including Fort Worth, Texas; Georgia; Seattle; and New York. The FBI has received “hundreds” of tips, she said.

Ron Machen, the US attorney for the District of Columbia, is also investigating the course of events that led to the shooting, but said it will be weeks or possibly months until a fuller story emerges. “We’re not going to stop until we get answers,” he said. “What caused this individual to kill so many men and women? How did he get those weapons?”

Metropolitan Police Department Chief Cathy Lanier did offer a bit of new information about how officers responded to the initial calls of gunfire. Within two minutes of receiving the first call, about 8:20 AM, MPD had two units, armed with AR-15 rifles, en route to the base. She said at least five police units were through the yard’s gates within four or five minutes, and two units reached Building 197, where the rampage went down, seven minutes after the first call.

“The actions they put in were unbelievable,” Lanier said of the officers from several local and federal agencies who were dispatched. It took multiple engagements to take down the shooter. Lanier said Alexis was killed between 30 and 60 minutes after police first arrived.

Update 6 PM: Alexis purchased a shotgun from Sharpshooters Small Arms Range in Lorton, Virginia, according to a statement Tuesday afternoon from a lawyer representing the gun shop. Alexis bought a Remington 870 and two boxes of ammunition—24 shells—after renting a rifle to use at the store firing range, said the store’s lawyer, J. Michael Slocum. Slocum says the shop followed all the proper procedures in selling the shotgun.

“In accordance with federal law, Mr. Alexis’ name and other applicable information, including his state of residency, was provided to the federal NICS system and he was approved by that system,” Slocum said. “After the terrible and tragic events at the Navy Yard, the Sharpshooters was visited by federal law enforcement authorities, who reviewed the Range’s records, including video and other materials. So far, as is known Mr. Alexis visited the Range only once, and he has had no other contact with the Range, so far as is known.”

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Benjamin Freed joined Washingtonian in August 2013 and covers politics, business, and media. He was previously the editor of DCist and has also written for Washington City Paper, the New York Times, the New Republic, Slate, and BuzzFeed. He lives in Adams Morgan.